Show Multiple Weights
You cannot see simultaneously two weight influences. I don't know how that can be possible to see on the same mesh as they may overlap.

Though, even if that does not answer the question, you can use the "multi paint" option in the drawing toolshelf.



As indicated in the tool tip, that allows to paint over the selected bones and maintaining their relative influence.

So the paint will affect these bones so that they stay relatively the same between them but change relatively to the other (non selected) bones.

But doing that you cannot compare them, as, as you can see the color shown here is homogeneous.

What you can do, though is opening two views, one with your model, one with UV image editor and proceed to an opengl rendering: